A method for programming a nonvolatile memory cell having a floating gate type of transistor includes a conventional hot electron injection method to inject hot electrons from the high-potential side (drain side) of a programming voltage applied between the drain and source, to a floating gate, and a source-side injection method to inject hot electrons from the low-potential side (source side) to the floating gate. While each of them is the well-known programming method, the source-side injection method has advantages of being high in electron injection efficiency by about three digits, as compared with the conventional hot electron injection method, and of being able to implement high-speed programming.
The nonvolatile memory cell to perform the programming by the source-side injection method has a split-gate structure in which a first gate electrode formed on the source side, and a second gate electrode formed on the drain side are arranged in series in a channel region formed between a pair of source and drain impurity diffusion layers (refer to the following patent document 1, for example).
As shown in FIG. 13, in general, the second gate electrode serving as a floating gate is formed of a first polysilicon layer, an oxide film is deposited thereon, a second polysilicon layer is deposited thereon, and the first gate electrode is formed of it by photolithography. At this time, the first gate electrode and the second gate electrode are overlapped, and a distance between the gate electrodes in a source-drain direction is defined by a film thickness of the oxide film between the first gate electrode and the second gate electrode.    Patent document 1: Japanese Patent No. 2862434